The invention refers to power plants that include high temperature fuel cells, in which hydrocarbon fuel is used as initial fuel.
High temperature fuel cells efficiently convert fuel energy into electric power. An electrochemical reaction involving the fuel (usually a mixture of hydrogen and carbon monoxide) and air takes place in the fuel cell, which produces electric power. However, not all of the energy in the fuel is converted into electric power; energy is lost for two reasons. First, the efficiency of the electrochemical conversion is no greater than 0.6xe2x80x94the rest of the chemical energy turns to heat. Second, the oxidation products (carbon dioxide and water vapors), which are generated in the result of fuel cell reaction, dilute the flow of fuel along the length of fuel channels. The partial pressures of the fuel components decrease, reducing the reaction rate.
To counteract this, more fuel than can possibly be consumed is fed into the fuel cell, which is why there is always unused fuel in the output flow. The remnants of this fuel, together with the oxidation products, are commonly utilized in co-generation engines which produce additional electric power. Attempts are also made to utilize the heat generated by fuel cells, e.g. in power plants to heat fuel or air that is supplied to the fuel cell.
Power plants are known in which unconverted fuel energy in a high-temperature fuel cell is utilized by means of a gas turbine, as in the inventions described in Japanese patents (1) No. 63119163 xe2x80x9cFUEL CELL GENERATING SYSTEMxe2x80x9d, priority datexe2x80x94Nov. 7, 1986, publication datexe2x80x94May 23, 1988, IPC H01M 8/06, (2) Japan patent No. 4065066 xe2x80x9cFUEL CELL AND CARBON DIOXIDE GAS FIXED COMPOUND POWER GENERATION METHODxe2x80x9d, priority datexe2x80x94Jul. 5, 1990, publication datexe2x80x94Mar. 2, 1992, IPC H01M 8/06, and (3) Japan patent No. 1021463 xe2x80x9cDEVICE AND METHOD OF REPRODUCING ELECTRICITY AND BY-PRODUCING HYDROGEN xe2x80x9d, priority datexe2x80x94Dec. 19, 1996, publication datexe2x80x94Aug. 11, 1998, IPC H01M 8/06 and (4) U.S. Pat. No. 5,541,014 xe2x80x9cINDIRECT-FIRED GAS TURBINE DUAL FUEL CELL POWER CYCLExe2x80x9d, priority datexe2x80x94Oct. 23, 1995, publication datexe2x80x94Jul. 30, 1996, IPC H01M 8/06. These are designed as stationary power plants for electrical generation stations. The use of gas turbines in power plants with fluctuating loads (for example, those used in vehicles) is less effective. In addition, low power turbines (under 10-20 kW) have low efficiencies.
The power plant described in (5) U.S. Pat. No. 5,968,680, xe2x80x9cHYBRID ELECTRICAL POWER SYSTEMxe2x80x9d, priority datexe2x80x94Sep. 10, 1997, publication datexe2x80x94Oct. 19, 1999, IPC H01M 8/06, comprises a fuel cell, combustion chamber intended for combusting the fuel remnants at the fuel cell outlet, and a turbine. The turbine turns an electric generator. The power plant also comprises a reformer, which converts hydrocarbon fuel into a fuel for the high-temperature fuel cell. It is possible to vary the power of the power plant by regulating the fuel supply to the fuel cell. To increase the power output of the turbine, additional fuel can be supplied to the combustion chamber. This power plant is intended for operation in stationary power generating units and is not efficient under fluctuating loads.
The present invention solves the problem of how to maximize energy utilization of the hydrocarbon fuel in a power plant with a high temperature fuel cell, which is capable of performing efficiently under fluctuating loads. Maximum utilization of the fuel energy makes it possible to reduce the power and size of the fuel cell used in the power plant.
The first embodiment of the power plant comprises a reformer which converts hydrocarbon fuel into a fuel mixture comprising mainly of hydrogen and carbon monoxide; a high temperature fuel cell comprising an air duct with an inlet and an outlet and a fuel channel with a corresponding inlet and outlet; a distributor with one inlet and two outlets; a combustion chamber with fuel inlet, air inlet and outlet; and a volume expansion engine with an inlet for supplying the working fluid. The outlet of the reformer is connected to the inlet of the fuel channel of the high temperature fuel cell. The outlet of the fuel channel of the high temperature fuel cell is connected to the inlet of the distributor; one outlet is connected to the fuel inlet of the combustion chamber, while the other outlet is connected to the inlet of the reformer. The outlet of the air duct of the high temperature fuel cell is connected to the air supply inlet of the combustion chamber, while the outlet of the combustion chamber is connected to the volume expansion engine.
Hydrocarbon fuel is fed to the reformer where it is converted into a mixture of hydrogen and carbon monoxide. The hydrogen and carbon monoxide are then fed to the fuel channel and oxygen (air) is fed to the air duct of the high temperature fuel cell. The chemical energy of the air (oxygen), hydrogen and carbon monoxide is converted into electric energy via electrochemical reactions. Unreacted hydrogen and carbon monoxide, together with the oxidation products, are then fed to the combustion chamber. Air containing unused oxygen is also supplied to the combustion chamber.
Unlike known systems, a quantity of hydrogen and carbon monoxide, together with oxidation products, carbon dioxide and water vapor, is again fed to the reformer inlet. The increased concentration of carbon dioxide and water vapor in the reformer increases the efficiency of the reformer, as well as the output of hydrogen and carbon monoxide.
The outlet of the combustion chamber is connected to the volume expansion engine. Engines of this type are better suited to operation under varying loads than turbines. They can also operate effectively at low power. The hot combustion gases perform mechanical work as a result of their expansion in the volume expansion engine. This mechanical energy can be converted into extra electric power by means of electric generator.
The distributor distributes the output from the outlet of the high temperature fuel cell either to the combustion chamber (power output from the volume expansion engine increases rapidly) or back to the reformer (efficiency of fuel utilization in the fuel cell increases). The distributor provides better power plant performance under varying loads.
In one exemplary embodiment according to the principles of the present invention, the combustion chamber is connected to the reformer via a heat exchanger, which heats the reformer. This design provides two benefits. First, high temperature heat from the combustion chamber intensifies the hydrocarbon fuel conversion processes in the reformer. Second, transferring some of the heat to the reformer reduces the temperature of combustion products. This, in turn, reduces the demand on part of volume expansion engine; the lower temperature does not requires expensive alloys and therefore reduces manufacturing costs.
The volume expansion engine can be connected to an electric generator to produce additional electric power. To do this, fuel is fed to the fuel cell at a higher rate than it can be consumed. The excess fuel is then burned in the combustion chamber or is fed back to the reformer. The distributor regulates this ratio to maximize efficiency of the power plant at a given load.
Also, a heat exchanger can be installed on the high temperature fuel cell to increase the temperature of the fuel fed to the reformer and air supplied to the high temperature fuel cell. This increases the efficiency of the power plant.
Also, to overcome aerodynamic losses, a pump can be installed between the distributor and reformer inlet to increase the pressure of the products supplied from the output of the fuel channel of high temperature fuel cell.
A second exemplary embodiment of the power plant comprises a reformer for converting hydrocarbon fuel into a fuel mixture consisting mainly of hydrogen and carbon monoxide; a high temperature fuel cell comprising an air duct with an inlet and an outlet and a fuel channel with a corresponding inlet and outlet; a distributor with one inlet and two outlets; a combustion chamber with fuel supply inlet, air supply inlet and outlet; and a volume expansion engine comprising an inlet for supplying the working fluid, one regulating valve, the inlet of which is connected to air inlet and another regulating valve, the inlet of which is connected to a hydrocarbon fuel inlet.
The outlet of the reformer is connected to the inlet of the fuel channel of the high temperature fuel cell. The outlet of the fuel channel of the high temperature fuel cell is connected to the inlet of the distributor; one outlet is connected to the fuel inlet of the combustion chamber, while the other outlet is connected to the inlet of the reformer. The outlet of the air duct of the high temperature fuel cell is connected to the air supply inlet of the combustion chamber, while the outlet of the combustion chamber is connected to the volume expansion engine. The outlet of the first regulating valve of the volume expansion engine is connected to the air supply inlet of the combustion chamber, while the second regulating valve is connected to the additional fuel supply inlet of the combustion chamber.
The second embodiment of the power plant increases the rate at which the power output of the volume expansion engine can be increased. This makes it possible to improve the operation of the power plant under varying loads where it is necessary to rapidly increase the power output. Otherwise, operation of the second embodiment of the power plant is analogous to the first.
In the specific case of the second embodiment, the combustion chamber is connected to the reformer via a heat exchanger which transfers heat to the reformer. The volume expansion engine can be mechanically connected to an electric generator. The high temperature fuel cell can comprise an additional heat exchanger connected to a device for heating fuel supplied to the reformer, and a device for heating air supplied to the fuel cell. An additional pump can be installed between the distributor and the reformer inlet.
Power plants designed according to the first and second embodiments offer the most efficient utilization of hydrocarbon fuel energy.